A variety of techniques and equipments have been used to remove the limbs of trees prior to, or after, the felling of the trees. Perhaps the most fundamental method of removing limbs has been to use axes, manual saws or chain-saws. Such equipment can be used effectively in confined spaces and on steeply-sloping hillsides, where it is difficult to provide and operate mechanical delimbing equipment. The manual removal of branches is both labour intensive and time consuming. For this reason, mechanical delimbers have been developed and have been widely adopted in pine plantations. Thus manual removal of branches is now used only to a limited extent in pine plantations. However, there has been no satisfactory mechanical limb removal equipment for use with eucalypts.
In more detail, to remove the limbs from standing pine (and similar) trees in a plantation, "high pruners" and "tree shears" have been developed. A useful summary of the equipment available and used for pruning is provided in the paper by P. Wilkes and L. J. Bren, entitled "Radiata Pine Pruning Technology", published in Australian Forestry, volume 49, 1986, pages 172 to 180. This paper includes a reference to the equipment known as the "Tree Monkey", which is an automated pruner that, in use, is clamped around the base of a tree, from which position it spirals upwards on the tree trunk. A chainsaw, mounted on the equipment with its chain guide or bar vertical, is used to remove the limbs of the tree as they are encountered. When the tree trunk diameter reaches a predetermined minimum value, the "Tree Monkey" reverses direction and spirals back down the trimmed trunk. Unfortunately, this equipment is not noted for its reliability, and is not used on eucalypts, where often the limbs are present only near the top of the tree.
When a tree has been felled, the limbs may be removed by using equipment which effectively wraps three or more curved knives around the trunk of a tree. One of these knives is fixed; the other knives are moveable. The assembly of knives is then driven along the trunk, cutting through any branch encountered by a knife. The disadvantages of this approach to delimbing are (a) that a high energy input is required to drive the knife blades through the branches and along the trunk; (b) the wrap-around arrangement acts as a brake to the movement of the assembly (and thus contributes significantly to the energy input requirement); and (c) the inability of the curved knives to provide a true 360.degree. close coverage of the trunk, with the result that stub ends of the severed limbs inevitably remain on the trunk.
Yet another example of the prior art approach to delimbing is described in the aforementioned specification of Australian patent application No. 24,114/88 namely the inclusion of a short chain-saw or circular saw in the tree felling assembly embodiment of the debarking equipment described in that specification. The short chain-saw or the circular saw is mounted above the debarking equipment, with the plane of the bar of the short chain-saw, or the plane of the circular blade, parallel to the direction of movement of the log or trunk through the debarking equipment. Using this equipment, branches are out from the trunk, close to it, as the log or trunk is moved past the chain-saw or circular saw.
A disadvantage of all of the mechanical prior art delimbing equipment is that although limbs can be removed fairly close to the trunk when the equipment is used, nevertheless there is always a stub end of the limb which remains, attached to and projecting from the trunk. These stub ends reduce the packing density of logs and trunks on timber forwarders. They also reduce the effectiveness of debarking equipment--and on occasions cause a log to jam in debarking equipment. Thus the presence of short stub ends of branches is economically disadvantageous to the logging industry.